Selenocysteine, commonly referred to as the twenty-first amino acid, is incorporated into at least 25 human proteins. Natural co-translational incorporation of selenocysteine (Sec) into proteins proceeds by a recoding process so that upon encountering the UGA codon in the messenger RNA the ribosome knows to recognize it as Sec instead of Stop. This process requires three components: (i) the aminoacyl-tRNA carrying selenocysteine, Sec-tRNASec; (ii) the specialized elongation factor, SelB, carrying Sec-tRNASec to the ribosome, and (iii) the SECIS element, an RNA secondary structure of the mRNA just downstream of the UGA codon, that interacts with the SelB●Ser-tRNASec complex (Böck, A, Thanbichler, M, Rother, M & Resch, A (2005), eds Ibba M, Franklyn C S, & Cusack S (Landes Bioscience, Georgetown, Tex.), pp 320-327; Yoshizawa, S & Böck, A (2009) Biochim Biophys Acta 1790:1404-1414). Additionally, in order to protect the integrity of this recoding process, Sec-tRNASec is not recognized by the general elongation factor EF-Tu because of the presence of three base pairs that act as antideterminants (Rudinger, J, Hillenbrandt, R, Sprinzl, M & Giegé, R (1996) EMBO J 15:650-657. Sec-tRNASec cannot be accommodated during normal translation because it is not an acceptable substrate for EF-Tu, and the SelB●Sec-tRNASec complex will not decode in-frame UGA codons in absence of the SECIS.
Insertion of selenocysteine into a recombinant protein, for example, substitution of a naturally occurring cysteine residue for selenocysteine, can alter the function of the protein. Substituting one or more naturally occurring Cys residues in the active site of an enzyme with a Sec can increase the activity of this enzyme. Diselenide bonds have very low redox potential. Therefore, replacing disulfide bonds with diselenide or selenocysteine-cysteine bonds can lower dosage, increase half-life, increase stability, reduce toxicity, alter pharmacokinetics, change folding properties, or combinations thereof of the recombinant selenocysteine containing protein relative to a reference protein without selenocysteines, such as a naturally occurring counterpart.
However, due the presence the SECIS element as an integral part of the open reading frame (within the mRNA) encoding the protein that harbors Sec in its sequence, it is not possible to insert Sec into proteins by a standard mutational scheme or in the construction of random mutagenic libraries, and production of Sec proteins is limited to costly and inefficient methods of protein synthesis. Accordingly, there is a need for alternative methods of manufacturing selenocysteine containing polypeptides.
It is an object of the invention to provide compositions and methods for recombinant expression of proteins engineered to include one or more selenocysteine residues without the requirement of a SECIS in the mRNA encoding the protein.
It is a further object of the invention to provide non-naturally occurring proteins including one or more selenocysteine residues.